Foam-tank debris will be key concern for Discovery's second test flight

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The countdown clock begins ticking toward the launch of the space shuttle Discovery at NASA's Kennedy Space Center in Florida on Wednesday. The count starts at T-minus-43 hours, with multiple built-in holds. But because the Apollo-era clock can only go up to 24 hours, the display started out at 19 hours - as in "one day and 19 hours."

A year ago, the shuttle flight known as STS-121 might have been seen as little more than a rerun of NASA's first post-Columbia mission — a second chance to test the safety modifications made after the 2003 tragedy.

But when shuttle Discovery lifted off for last summer's "return to flight" mission, NASA's managers were surprised to see a piece of foam insulation breaking off the external fuel tank and flying past the shuttle's wing. NASA believes it was just such a mishap that led to Columbia's destruction — and so STS-121 was grounded for months more while the tank went through another round of redesign.

Today, with NASA beginning the countdown toward STS-121's scheduled Saturday launch to the international space station, the mission is much more than a summer rerun. If anything, this year's flight of Discovery is even more crucial than last year's flight of Discovery.

Testing the changes made over the past year is the top task, of course. After extensive wind-tunnel testing, NASA engineers decided to remove the source of last year's foam-debris problem — a 34-pound (15-kilogram), hand-applied section of insulation known as the protuberance air load ramp.

NASA put components and scale models of the redesigned tank through weeks of extensive aerodynamic testing in wind tunnels.

"The results just came back, and the conclusion is, we have good margin and we're good to fly with it," Discovery commander Steve Lindsey told NBC News earlier this month. "But nevertheless, it's probably the largest change to the exterior of the vehicle, the whole shuttle system, we've made since the beginning of the program."

NASA has also addressed some of the other issues that caused so many headaches last year — throwing out a glitch-prone batch of fuel-tank sensors, replacing 15,000 potentially loose gap fillers wedged in between Discovery's protective tiles, and even fixing rubber seals on the windows.

But shuttle program manager Wayne Hale admitted that "we still have areas to improve," particularly when it comes to the flyaway foam problem. In fact, several safety officials recommended holding off on Discovery's launch until engineers could finish fixing another potential problem area: 34 aerodynamically shaped foam ramps meant to protect the supercooled tank's fittings from flying chunks of ice.

Taking a risk on a flight test
In the end, NASA Administrator Mike Griffin sided with those who said the changes made so far needed to be tested sooner rather than later — a decision that met with the Discovery astronauts' approval.

"In my opinion, we made this big change — we ought to go flight-test that change as we develop the new change for the ice frost ramp," said Lindsey, a veteran test pilot.

Part of Griffin's reasoning was that even if flying foam did so much damage to Discovery that it couldn't return home safely, Discovery's crew could shelter themselves on the space station for the several weeks it would take to launch a sister shuttle, Atlantis, on a rescue mission. But the NASA chief admitted that such a scenario could deal a fatal blow to the shuttle program — even if no lives were lost.

"If we were to lose another vehicle, I would tell you right now that I would be moving to figure out a way to shut the program down," Griffin told reporters earlier this month. "I think at that point we're done."

Failure vs. success
If STS-121 is a failure, that would raise grave questions about the future of the international space station as well: Since the Columbia tragedy, the job of resupplying the station has largely fallen to unmanned Russian cargo ships — but currently, only the shuttle can bring up large pieces of the station that still have to be installed, such as the Japanese and European laboratory modules.

But if STS-121 is a success, that would clear the path for an ambitious schedule of 16 space station construction missions over the next four years — and perhaps a final servicing stop at the Hubble Space Telescope, which experienced a worrisome glitch
just this month
. NASA officials say they would continue to make safety improvements in the shuttle system all the way up to the fleet's retirement, now scheduled for 2010.

So how do you judge success? It's not just a matter of getting up to orbit and back down to Earth in one piece. NASA's agenda for Discovery's 12-day mission also includes trying out some safety procedures that aren't yet quite ready for prime time, resupplying and repairing the space station, doing a bit of space science — and generally adjusting to the "new normal" for post-Columbia shuttle flights.

Trying new safety tricks
Besides the tank redesign, one of the biggest safety improvements has to do with how Discovery will get to orbit. The flight plan calls for the shuttle to throttle back earlier as it passes through the thickest region of the atmosphere, thus reducing the strain on the tank's insulating foam.

Lindsey compared it to the difference in force that you feel when you stick your hand out the window of a car traveling at 45 mph as opposed to 60 mph.

"We want to go through that thickest part of the air as slow as we possibly can, so that we can gain most of our speed after we're out of that air and past that danger zone," he explained. The flight profile, known as "Low Q," had been flown by the shuttle until about 10 missions ago, when it was changed to enable the shuttle to carry more weight.

Once Discovery is in orbit, the crew will conduct a thorough inspection using a sensor-tipped extension boom attached to the shuttle's robot arm — just as was done during last year's flight. The space station's crew will make a high-resolution camera survey just before the shuttle docks.

After docking, spacewalkers Piers Sellers and Mike Fossum will take turns standing at the very end of the extension boom and going through the kinds of motions they'd have to make to inspect or even repair the shuttle. If the shuttle can conserve enough fuel to extend the mission to a 13th day, the spacewalkers will also test tools for checking and fixing cracks in the shuttle's reinforced panels.

Just before and after undocking from the station, the shuttle will be surveyed again, to catch any damage that might have been done by micrometeoroids or orbital debris. This round of inspections is a new addition to the shuttle flight routine.

Getting the station back up to speed
For the space station's crew, Discovery is like a big delivery van coming in, and a recycling truck going out. About two and a half tons of supplies will be transferred to the station from an Italian-built cargo container in the shuttle's payload bay, and almost as much used equipment and trash will be taken from the station and stowed in the container.

Some of the equipment being delivered will come in handy for the eventual expansion of the station. For instance, a new U.S.-built oxygen generator will take the load off the sometimes-balky Russian generators and provide enough air for up to six long-term station crew members. Discovery is also bringing up a top-of-the-line lab freezer and a new incubator for plant growth experiments, as well as an upgraded exercise cycle and air conditioner.

Discovery's spacewalkers are scheduled to fix a power cable system for the space station's robotic rail car — in the process, replacing a backup cable that was cut accidentally last December. The rail car system will have to be working again before NASA moves ahead with the next construction mission, currently scheduled for September.

The spacewalkers will also install a spare part for the station's cooling system.

The highest-profile addition is German astronaut Thomas Reiter, the European Space Agency's first long-term space station resident. He'll also be the first astronaut to join one station crew (Expedition 13) and stay on through a crew rotation — thus providing an extra measure of continuity for space station operations.

Reiter's presence will boost the station's available crew time by 50 percent, allowing the crew to rise out of its current maintenance mode. Mission planners figure that keeping the station running takes up almost all the time of two residents. Having three on board again "can definitely increase the scientific output" of the station, Reiter told reporters.

Reiter has been assigned 19 ESA experiments for his five- to six-month tour of duty, and he also plans to smooth the way for next year's scheduled arrival of the ESA's
Columbus lab module
.

Getting used to NASA's new normal
This summer's shuttle mission may well set the tone for all the shuttle missions to come: Each mission will feature incremental improvements in safety, tempered with an increased awareness that things will go wrong anyway. Hale said he was certain "we will have a dozen to 100 in-flight anomalies" during the course of Discovery's flight — and he worried about resolving those anomalies in time for Atlantis' launch window in late August and early September.

"The long pole is being able to investigate whatever anomalous things happen in flight," he told reporters.

Discovery's crew seems to be more sensitized to the continuing risks as well. Last year, STS-114 commander Eileen Collins said she was disappointed to see that flying foam still posed a problem. "We thought we had that problem licked," she told reporters during the mission.

This month, Lindsey told MSNBC.com that the tank-foam setback was part of "the nature of the business we're in."

"When we get a setback like that, and we get them all the time in this business, we have to go deal with it," he said. "We get over being upset about it, and we focus on fixing it. And I think that's what the program has done for the last year."

The 12 orbiting astronauts on the International Space Station were hailed by the chief on Thursday — and also got a humorous scolding for keeping their android crewmate locked up in its packing for so long.